Get the entire assembly in C#

I'm trying to eliminate a call from my API in which the end-user has to pass Assembly.GetExecutingAssembly() in their code, currently my code looks lke the following.

public static Dictionary<int, Packet> Packets { get; private set; }

public static void Init()
{
    Assembly assembly = Assembly.GetExecutingAssembly();

    foreach (Type type in assembly.GetTypes())
    {
        if (type.BaseType == typeof(Packet))
        {
             ...
        }
    }
}

However this only gets all of the classes that are available in the API.dll, and none of the classes from the project that's using the .dll are included,

However if I pass it as a paramter, like so:

 public static void Init(Assembly assembly) {}

it loads everything just fine, is this intended behaviour, is there a way to do this all in the background without the end-user ever having to worry about the Assmebly call?

In a nutshell - I have a .dll file that people will be using in their projects, I want to get all of the classes from the project that's using the .dll, not all of the classes inside of the .dll